The invention relates to a piston-cylinder unit, in particular, for the hydraulic actuation of a clutch in a vehicle, with a piston, a cylinder, and a sealing material, wherein the piston-cylinder unit is fluidically linked to a fluid circuit, wherein the piston is displaceable axially in the cylinder, and wherein the sealing material is arranged sealingly between the piston and the cylinder. The invention also relates to a method for operating the piston-cylinder unit.
For the actuation of clutches, hydraulic actuators can be used that allow a targeted closing and opening of the clutch. Such hydraulic actuators are regularly formed by piston-cylinder units. These piston-cylinder units have a cylinder and a piston that is displaceable inside the cylinder. For sealing, at least one sealing element is arranged between the piston and the cylinder. By shifting the piston relative to the cylinder, a work fluid can be pumped or a pressure can be built up, wherein an adjustment of an adjustment element, for example, a hydraulically actuated clutch or a brake, can be realized. Such a piston-cylinder unit has become known, for example, from DE 195 23 215 A1.
Temperature changes or other influences can cause a change of the volume of the work fluid. This change in volume can have a negative effect on the operation of the piston-cylinder unit. Therefore, the change in volume must be equalized. Such a volume equalization is enabled, for example, by a so-called snifting process. During the snifting process, the change in volume of the work fluid is equalized by a feeding of work fluid from a storage volume or by the discharging of work fluid into a storage volume. The feeding or discharging of the work fluid can be achieved here, for example, by a hole or a groove arranged in the piston and/or in the cylinder. The snifting process is triggered in that the piston-cylinder unit is moved into a predetermined position, which enables the regulated fluid exchange.
In the prior art, solutions are known in which two piston-cylinder units are connected to each other such that the actuation of one piston-cylinder unit actuates the corresponding second piston-cylinder unit. In this way, one of the piston-cylinder units has the slave cylinder and the other piston-cylinder unit has the master cylinder, wherein the actuation of the piston in the slave cylinder is caused by means of the piston of the master cylinder.
In order to enable the most dynamic control possible, the goal of such piston-cylinder units is to actuate as quickly as possible in order to quickly open or close a clutch actuated by at least one piston-cylinder unit. Temperature effects can cause, among other things, an increase in friction that has the result that the adjustment of a piston-cylinder unit takes place more slowly. The different adjustment rates can lead to the formation of an underpressure in one of the piston-cylinder units. This underpressure can cause leaks due to the seals being lifted from their sealing surfaces. This can lead to the undesired suction of air or work fluid into the piston of the affected piston-cylinder unit. In both cases, in particular, the characteristic curve is negatively affected with respect to the operating path of the piston-cylinder unit. An underpressure can also be produced in an individual piston-cylinder unit.
If the affected piston-cylinder unit is used for controlling a clutch, the coupling characteristic curve of the clutch is also influenced with respect to the operating path of the piston-cylinder unit, which can cause losses in comfort, because the expected torque to be transmitted to the clutch based on the torque control no longer agrees with the actually transmitted torque.